After intense brainstorming on how to make a pattern for these pants, I made a muslin. I used the Bella pattern from Burda Style, and altered it a lot to get the collar detail on the waist. Here's how the muslin looks. The wrinkles on the crotch are there because the muslin was a bit tight, but I'll fix this while sewing.

I'll use a gorgeous gray wool I have in my stash, that was bought before I was even born, and it was supposed to become a suit for my dad. Keep your fingers crossed for me, I'm gonna sew tonight! :-)